That is not correct, the question was locked by a Moderator with a link to a post by "an official representative of Xbox Support". [thread=2241296]This is the old AT:CG thread[/thread], for reference.He always posts in these threads and uses a random xbox forum poster as proof (not a MS employee, just some random guy who says that it is not allowed)
Despite the official line, there is plenty of anecdotal evidence that MS does not ban for hard drive swapping, "generic" drives, or cloned firmware.